Hollow core optical fiber and a laser system

ABSTRACT

A hollow core photonic crystal fiber (PCF) including an outer cladding region and seven hollow tubes surrounded by the outer cladding region. Each of the hollow tubes is fused to the outer cladding to form a ring defining an inner cladding region and a hollow core region surrounded by the inner cladding region. The hollow tubes are not touching each other, but are arranged with distance to adjacent hollow tubes. The hollow tubes each have an average outer diameter d2 and an average inner diameter d1, wherein d1/d2 is equal to or larger than about 0.8, such as equal to or larger than about 0.85, such as equal to or larger than about 0.9. Also, a laser system.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/213,653, filed on Mar. 26, 2021, which is a continuation of U.S.application Ser. No. 16/701,690, filed on Dec. 3, 2019, which is acontinuation of U.S. application Ser. No. 16/064,751, filed on Jun. 21,2018, which is a U.S. National Stage of International Application No.PCT/DK2016/050460, filed on Dec. 22, 2016, which claims the benefit ofDanish Application No. PA 2016-70263, filed on Apr. 27, 2016, and DanishApplication No. PA 2015-70877, filed on Dec. 23, 2015. The entirecontents of each of U.S. application Ser. No. 17/213,653, U.S.application Ser. No. 16/701,690, U.S. application Ser. No. 16/064,751,International Application No. PCT/DK2016/050460, Danish Application No.PA 2016-70263, and Danish Application No. PA 2015-70877 are herebyincorporated herein by reference in their entirety.

TECHNICAL FIELD

The invention relates to a hollow core photonic crystal fiber and alaser system comprising such hollow core optical fiber.

BACKGROUND ART

Hollow core photonic crystal fibers (PCF) have been known for many yearsand in some applications are very attractive to use compared to solidcore PCF due to their very low propagation loss, low nonlinear effectsand high damage threshold.

A hollow-core fiber is an optical fiber which guides light essentiallywithin a hollow region, so that only a minor portion of the opticalpower propagates in solid fiber material surrounding the core. Thehollow region may be filled with air or any other gas or it may beevacuated to have a very low gas pressure.

Over the years several types of hollow core PCFs have been developed,such as hollow-core photonic bandgap fibers e.g. as described in U.S.Pat. No. 6,892,018, Kagome design type fibers e.g. as described in U.S.Pat. No. 8,306,379.

The kagome type fiber guides light by means of an anti-resonant effectand allows for a substantially broader spectral transmission than thatachieved in photonic bandgap fibers.

In recent years such hollow core anti-resonant fibers (ARFs) have beenstudied intensively in particular with the aim of further improving thefiber with respect to low loss and broad spectral transmission.

In an attempt to reduce the attenuation Kolyadin et al, Optics Express,Vol 21, pages 9514 to 9519, 2013 suggested a hollow core fiber with ahollow core surrounded by eight identical capillaries. It was concludedthat to reduce loss, a larger core size is preferred and by increasingthe number of capillaries in the cladding a reduced loss could beachieved.

Another drawback of prior art hollow core anti-resonant fibers is thatthey are not purely single mode waveguides because they also supportsome higher-order modes (HOMs).

Since these HOMs often have a relatively low loss, it is very difficultto launch a pure LP01 mode without HOM contamination and further HOMscan be excited by bending or external stress.

In an attempt to solve this problem and to achieving the highestpossible suppression of HOMs while maintaining a reasonably low loss forthe LP01 mode, a modified fiber design was suggest in “Broad-bandrobustly single-mode hollow-core PCF by resonant filtering of higherorder modes” by Gunedi et al., Cornell university libraryarXiv:1508.06747 [physics.optics], 27 Aug. 2015. The ARR fiber describedin this article comprises a central hollow core (inner diameter D)surrounded by six evenly spaced and non-touching capillaries (in thefollowing also called hollow tubes) with a wall thickness t and an innerdiameter d, supported within a thick-walled supporting capillary. Thenon-touching glass elements had modes which were tailored to ensureresonant phase-matched coupling to higher-order core modes, causing themto leak at a very high rate into the supporting solid glass sheath. Itwas found that extremely high suppression of HOMs could be achieved overbroad wavelength bands where the fiber guides with a low loss, providedthe ratio d/D 0.68 and the walls of the tubes walls T were thin enoughi.e. t/D=0.01.

WO15185761 describes a similar anti-resonant hollow-core fibercomprising a first tubular, cladding element which defines an internalcladding surface, a plurality of second tubular elements which areattached to the cladding surface and together define a core with aneffective radius, the second tubular elements being arranged in spacedrelation and adjacent one of the second tubular elements having aspacing there between, and a plurality of third tubular elements, eachnested within a respective one of the second tubular elements. It wasconcluded that the optimal number of tubes for suppressing HOMs was six.

DESCRIPTION OF THE INVENTION

The object of the invention is to provide a hollow core PCF whichalleviates at least one of the drawbacks described above.

In an embodiment it is an object to provide a hollow core PCF which hasa high spectral transmission and a high suppression of HOMs.

In an embodiment it is an object to provide a hollow core PCF which hasa high single mode transmission efficiency and is substantiallyinsensitive to bending.

In an embodiment it is an object to provide a hollow core PCF which in asimple way can be designed to have a desired low loss transmission bandwith a bandwidth of at least about 50 nm and preferably with atransmission loss of less than about 50 dB/Km.

In an embodiment it is an object to provide a hollow core PCF which hasa transmission band comprising wavelengths in the range of from about400 nm to about 1200 nm.

These and other objects have been solved by the invention or embodimentsthereof as defined in the claims and as described herein below.

It has been found that the invention or embodiments thereof have anumber of additional advantages which will be clear to the skilledperson from the following description.

The phrase “radial distance” means distance determined in radialdirection from the center axis of the hollow core. The phrase “radialdirection” is a direction from the core center axis and radiallyoutwards.

The term “substantially” should herein be taken to mean that ordinaryproduct variances and tolerances are comprised.

The term “about” is generally used to include what is within measurementuncertainties. The term “about” when used in ranges should herein betaken to mean that what is within measurement uncertainties are includedin the range.

It should be emphasized that the term “comprises/comprising” when usedherein is to be interpreted as an open term, i.e. it should be taken tospecify the presence of specifically stated feature(s), such aselement(s), unit(s), integer(s), step(s) component(s) and combination(s)thereof, but does not preclude the presence or addition of one or moreother stated features.

Throughout the description or claims, the singular encompasses theplural unless otherwise specified or required by the context.

Diameters, thickness and other structural values are seen in a crosssectional view of the hollow core PCF unless otherwise specified orclear from the context.

The hollow core photonic crystal fiber (PCF) of the invention comprisesan outer cladding region and 7 hollow tubes surrounded by the outercladding region. Each of the hollow tubes is fused to the outer claddingto form a ring defining an inner cladding region and a hollow coreregion surrounded by the inner cladding region. The hollow tubes are nottouching each other. It has been found to be advantageous forsuppressing of undesired higher order modes that the hollow tubes eachhas an average outer diameter which is referred to as d2 and an averageinner diameter which is referred to as d1. The ration d1/d2 is equal toor larger than about 0.8.

The average outer diameter d2 and/or the average inner diameter of therespective hollow tubes may be equal or different from hollow tube tohollow tube

In an embodiment d1 and/or d2 is/are substantially identical for atleast three of the hollow tube, such as for all of the seven hollowtubes.

According to the invention and against all teaching of prior art hollowcore PCF it has surprisingly been found that a hollow core PCF with anon-touching inner cladding structure where there are seven non-touchingtubes provides a highly increased suppression of higher order modes(HOMs) than the above described hollow core PCF with six 6 non-touchinginner cladding tubes. In particular—and as shown in the example—it hasbeen found that the hollow core PCF of the invention with sevennon-touching tubes has an increased suppression of HOMs with azimuthalnumber above 1, such a third order HOMs while simultaneously having lowloss in the fundamental mode of at least a transmission band below 2 μm,preferably below 1.5 μm.

The term “transmission band” is herein used to mean a band with abandwidth of at least about 10 nm, such as at least about 25 nm andpreferably at least about 50 nm or even at least about 100 nm oftransmission wavelength.

The term “low loss transmission band” means a transmission band forlight in its fundamental mode with a transmission loss of less thanabout 100 dB/km, preferably less than about 60 dB/km and more preferablyless than about 50 dB/km.

It has further been found that by increasing the ration d1/d2 the lowloss transmission band may be achieved at even lower wavelengths such asa low loss transmission band comprising wavelengths below 1200 nm, suchas below 1 μm, such as below 800 μm, such as below 600 my.Advantageously the ration d1/d2 is equal to or larger than about 0.85,such as equal to or larger than about 0.9.

The outer cladding comprises solid cladding material and is preferablyentirely of solid material.

In an embodiment the hollow tubes are arranged with a substantiallyequal distance to adjacent hollow tubes. Thereby a very high Gaussianbeam quality may be obtained. In an alternative embodiment the hollowtubes are arranged with different distances to adjacent hollow tubes. Inthe latter embodiment the variation in distance between adjacent hollowtubes is provided by having different outer diameters d2 of the tubes.Where the inner diameter d1 and/or the outer diameter d2 of the hollowtubes differs, the inner diameter d1, respectively the outer diameter d2is calculated as the average d1 and d2 values, unless the calculation isthe hollow tube thickness or otherwise specified. Accordingly the rationd1/d2 is determined by the average d1 and d2 values. It has been foundthat the optimal suppression of HOMs requires that the minimum distancebetween adjacent hollow tubes is relatively low and preferably less thanabout half the outer diameter d2 of the hollow tubes. On the other hand,the distance should not be too narrow because this may result in therisk of fully eliminating the distance along parts of or along the wholefiber length due to surface tension and material attraction duringdrawing of the fiber. To avoid such risk it is desired that the minimumdistance between adjacent hollow tubes is at least 0.01 times d2.

Advantageously the minimum distance between adjacent hollow tubes is atleast about 0.1 μm, such as at least about 1 μm, such as at least about2 μm, such as at least about 5 μm.

In an embodiment the minimum distance between adjacent hollow tubes isabout 5 μm or less, such as about 4 μm or less.

It has been found to be optimal that the center to center distance Abetween adjacent hollow tubes is between about 1.01*d2 and about 1.5*d2,such as between 1.05*d2 and 1.2*d2.

The hollow tubes advantageously have substantially parallel center axis.In practice the center axis of the hollow tubes may deviate slightlyfrom being straight due to process variations. The hollow tubes may forexample be helical around the core with a very long pitch, such as apitch of up to 1 km, such as from 1 cm to 100 m.

Due to the structure of the hollow core PCF the core may in principle bedesigned with any diameter of the core region. The core region isadvantageously substantially circular. In an embodiment the core regionis not circular, but rather oval or angular, such as substantiallypentagonal. The core diameter D is defined as the diameter of thelargest circle inscribed by the 7 hollow tubes.

Advantageously the hollow core region has a core diameter D of fromabout 10 μm to about 100 μm, such as from about 10 am to about 60 μm.

It has been found that the optimal core diameter D is scalable with thecentral wavelength of the transmission band of the hollow core PCF.

For a central wavelength of about 1.0 μm. the core diameter D isadvantageously as from about 20 am to about 50 μm, such as from about 25am to about 40 μm. The preferred core diameter D is directlyproportionally scaled with the central wavelength of the transmissionband of the hollow core PCF.

It has been found that the hollow core PCF may have a very high beamquality even where the core is relatively large. In an embodiment thebeam quality M² is about 1.75 or less, such as about 1.6 or less, suchas about 1.5 or even less.

In an embodiment the average outer tube diameter relative to the corediameter d2/D is from about 0.5 to about 0.75, such as from about 0.65to about 0.72, thereby a desired minimum distance between the hollowtubes and a very effective suppression of HOMs—in particular of 3^(rd)and 4^(th) HOMs—are ensured.

Further the mode quality of the beam obtained with a hollow core PCFaverage outer tube diameter relative to the core diameter d2/D is fromabout 0.5 to about 0.75 has shown to be very high.

It has been found the wall thickness of the hollow tubes largelyinfluences the low loss transmission band or bands of the hollow corePCF. In fact, the wall thickness t of relevance has been found to be thecore center facing region of the wall of the hollow tubes.

Thus it has been found that the wall thickness t (or the average valueof t) of the core center facing region of the hollow tubes mainlyaffects the location with respect to wavelengths of the low losstransmission bands. In general the hollow core PCF may have one orseveral low loss transmission bands, such as 1, 2, 3, 4 or even furtherlow loss transmission bands. Advantageously the hollow core PCF has atleast 3 low loss transmission bands, such as at least 4 low losstransmission bands. For obtaining one or more low loss bands below 1.2μm it is desired that the wall thickness t is up to about 2.1 μm, suchas up to about 1 μm, such as in the range from about 150 to about 350 nmor in the range from about 650 to about 850 nm or in the range fromabout 900 to about 2.1 μm.

For example it has been found that for a hollow core PCF of anembodiment of the invention the fiber has a low loss transmission at1030-1064 nm for a wall thickness t=150-350 nm, preferably 200-300 nm(low loss transmission band with highest wavelengths—band I), for a wallthickness t=650-850 nm, preferably 700-800 nm (2nd highest wavelengthslow loss transmission band—band II), and for a wall thickness of 5 timesfirst range for a third transmission band comprising 1030-1064 nm i.e.5*150 nm to 5*350 nm, seven times first range for a fourth transmissionband comprising 1030-1064 nm and so on).

Advantageously the hollow core PCF is designed such that the hollow coreguides light in the fundamental mode and comprises at least onewavelength between 200 nm and 4000 nm wavelength, such as between 400and 2000 nm, such as between 800 and 1600 nm, such as between 1000 and1100 nm, where the loss is less than about 1000 dB/km.

Advantageously the hollow core PCF has a low loss transmission band forthe fundamental mode with a transmission loss of less than about 100dB/km, preferably less than about less than 60 dB/km and more preferablyless than 50 dB/km comprising wavelengths in the interval between 1000and 1100 nm and preferably comprising at least the wavelengths 1030-1064nm

In an embodiment the hollow core PCF has a transmission loss for HOMs inits low transmission band for the fundamental mode of larger than about2000 dB/km, thereby ensuring an effective single mode transmission.

It has further been found that the hollow core PCF is substantiallyinsensitive to bending, for example it has been found that the hollowcore PCF of an embodiment of the invention has a low loss which is lessthan about 5%/km at 1030-1060 nm when coiled at a diameter of about 6cm.

Advantageously the wall thickness t of each of the hollow tubes issubstantially identical. Preferably the hollow tubes are substantiallyidentical and are arranged with identical distance to adjacent hollowtubes. Thereby the hollow core PCF becomes simpler to design for adesired wavelength transmission.

In an embodiment at least one of the tubes has a different wallthickness t than at least one other of the hollow tubes. Preferably 3 ofthe hollow tubes have a wall thickness and the remaining 4 hollow tubeshave another wall thickness. It has been found that by this arrangementthe hollow core PCF may become birefringent, largely depending on thedifference in wall thickness t and the relative position of the hollowtubes with larger and smaller wall thickness t.

In an embodiment at least one of the hollow tubes has a wall thicknesswhich is at least about 5% larger than the wall thickness of at leastone of the other hollow tubes, preferably at least one of the hollowtubes has a wall thickness which is at least about 10% larger than thewall thickness of at least one other of the hollow tubes.

In an embodiment each of the hollow tubes is substantially circular. Inpractice it is very difficult to obtain absolutely circular hollowtubes, first of all due to a slight deformation where the respectivehollow tubes are fused to the inner side of the outer cladding andsecondly because the hollow tubes may be attracted to adjacent hollowtubes during the drawing of the fiber.

In an embodiment each of the hollow tubes has a long inner diameterD_(l) and a short inner diameter D_(s) perpendicular to the long innerdiameter D_(l), wherein D_(l) is determined in radial direction from thehollow core center axis.

Advantageously the ration D_(s)/D_(l) is from about 0.5 to about 0.99,such as from about 0.8 to about 0.99.

In an embodiment the D_(s)/D_(l) ratio is larger than about 0.9, such aslarger than about 0.95 which in practice is simpler to produce.

In an embodiment the D_(s)/D_(l) ratio is smaller than about 0.95, suchas smaller than 0.9 which results in a relative large core diameter Dand thereby may add to reduce transmission loss for the fundamentalmode.

In an embodiment at least one of the hollow tubes comprises at least onenested sub tube arranged in the hollow structure of the hollow tubewherein the sub tube is fused to the hollow tube, preferably alignedwith the fusion to the outer cladding i.e. farthest from the coreregion.

The hollow tubes with one or more nested tubes may for example be asdescribed for the PCF of WO15185761 comprising second tubular elementswith third tubular elements nested within respective second tubularelements, with the difference that the hollow core PCF has 7 secondtubular elements with nested third tubular tubes.

The sub tube advantageously has an average outer diameter d_(sub), whichis substantially smaller than the average inner diameter d of the hollowtube. The average outer diameter d2 _(sub) is preferably up to about0.9*d2 of the hollow tube, such as up to about 0.9*d2, preferably theinternal sub tube is fused to the hollow tube at its maximal radialdistance to the core center axis.

In an embodiment at least one of the hollow tubes comprises one or morenodules arranged at a core center facing region of one or more of thehollow tubes, preferably the nodules are arranged at a boundary of thehollow core region, the nodules are preferably arranged to beanti-resonant at an operating wavelength, so that light of a fundamentalmode is substantially excluded from the nodules. Thereby an evenstronger confinement of the fundamental mode may be obtained to furtherreduce loss of light in the fundamental mode transmitted in the coreregion of the hollow core PCF.

The nodules may advantageously be in form of nodule-like, or bead-like,formation or a locally thicker region of the wall extending along atleast a part of the length of the fiber. Such nodules are sometimesreferred to as nodes or blobbies in the prior art optical fibers e.g. asdescribed in U.S. Pat. No. 7,321,712. The nodules and the remainingparts of the hollow tubes advantageously are of identical material.

The core region and/or the hollow tubes may in principle comprise anyfluid. Preferably the core region and/or the hollow tubes independentlyof each other comprise gas determined at standard ambient temperatureand pressure (SATP) as a temperature of 25° C. and an absolute pressureof exactly 100 000 Pa. Suitable gasses include air, argon, nitrogen ormixtures comprising any of the mentioned gasses. Optionally the hollowcore region and/or the hollow tubes independently of each other arevacummated (evacuated to have a very low gas pressure) or filled withpressurized gas.

In an embodiment the hollow core region and/or the hollow tubes arevacummated to a pressure of about 1 mbar or less, such as to a pressureof about 0.1 mbar or less, such as to a pressure of about of 0.01 mbaror less at standard temperature.

In an embodiment the hollow core region and/or the hollow tubes arepressurized to a pressure of up to 2 bars, such as up to about 1.5 barsat standard temperature.

The outer cladding region may in principle have any size provided thatit provides the hollow core PCF with a sufficient mechanical support forthe hollow tubes. In an embodiment the outer cladding region has anouter diameter of at least about 125 μm, 150 μm, such as at least about200 μm.

In general it is desired that the hollow core PCF is of one singlematerial which is preferably glass, more preferably silica, optionallydoped with a refractive index modifying dopant. In an embodiment one ormore, such as all of the hollow tubes are of doped silica and the outercladding region is of undoped silica. The dopant may for example includeindex changing materials such as F, Ge, P, B or combinations thereof.

Further it has been found that confinement loss may be reduced byproviding the outer cladding region to comprise a photonic bandgapstructure surrounding said inner cladding region.

The photonic bandgap structure may be provided by any means, e.g. byproviding the outer cladding region with an index grating comprisingconcentric rings with different refractive index and/or by includingmicrostructures having different index.

In an embodiment the outer cladding region comprises an outer backgroundmaterial having a refractive index N_(oc) and a plurality of inclusionshaving a refractive index different from the refractive index of thebackground material. The inclusions advantageously have a lowerrefractive index than the refractive index of the background material.The inclusions are preferably extending substantially parallel with thecore region. The inclusions may extend in a length section of the hollowcore PCF or in substantially the entire length of the hollow core PCF.

In an embodiment the plurality of inclusions in the outer backgroundmaterial is arranged in a cross-sectional pattern comprising at leasttwo rings of inclusions surrounding the inner cladding region, such asat least 3 rings, such as at least 4 rings of inclusions.

The phrase “ring of inclusions” refers to inclusions of the outercladding inclusions having substantially equal radial distance to thecore and being aligned in a ring configuration surrounding the core.Typically, a ring of inclusions is not fully circular, but rather isshaped with a number of soft angles, such as in a hexagonal shape.Preferably all the inclusions of a ring of inclusions are ofsubstantially the same size and preferably of same solid material, voidsand/or gas.

The background material may advantageously be silica, such as un-dopedsilica or doped silica. The dopant may for example include indexchanging materials such as F, Ge, P, B or combinations thereof.

In an embodiment the plurality of inclusions in the outer backgroundmaterial is arranged in a substantially hexagonal pattern.

In an embodiment the plurality of inclusions are voids or of gas, suchas air.

The diameter(s) of the inclusions is/are advantageously selected tominimize loss of a selected wavelength or a range of wavelengths. Thusthe diameter(s) may be selected to optimize for a desired transmissionprofile.

In an embodiment the inclusions have substantially identical diameters.

Advantageously the plurality of inclusions have an average diameter(d_(inc)) of up to about 2.5 μm, such as up to about 2 μm, such asbetween about 1.1 μm and 1.8 μm, such as between about 1.15 μm and about1.7 μm, such as between about 1.2 am and about 1.5 μm, such as about 1.3μm.

Also the distance(s) between the respective inclusions has shown to berelevant for optimizing (minimizing) confinement loss. In an embodimentfor optimizing confinement loss the plurality of inclusions are arrangedat a pitch (Λ_(inc)) of up to about 6 μm, such as up to about 5 μm, suchas up to about 4 μm, such as between about 2 μm and 4 μm.

In an embodiment the inclusions are arranged at a pitch (Λ_(inc)) of upto about 3.5 μm, such as up to about 3 μm, such as up to about 2.5 μm,such as between about 1.1 μm and 2 μm.

The invention also comprises a laser system for delivering laser lightto a user apparatus, the laser system comprising a laser light sourceand a fiber delivery cable for delivering light from the laser lightsource to the user apparatus, wherein the fiber delivery cable comprisesa hollow core PCF as described above.

The fiber delivery cable comprising the hollow core PCF may preferablyhave a length of up to 50 m, such as from about 0.3 m to about 20 m,such as from about 1 m to about 15 m.

Advantageously the laser light source is configured for generating laserlight pulses and is optically connected to the fiber delivery cable,preferably the laser light source is a femtosecond laser source.

The laser light source may in an embodiment be arranged for directlyfeed the light to the hollow core PCF e.g. by being fused to the hollowcore PCF. In an embodiment the laser light source is arranged forfeeding the light to the hollow core PCF via one or more opticalelements and/or via free space.

In an embodiment the laser light source has a pump duration of fromabout 30 fs to about 30 ps, such as from about 100 fs to about 10 ps.

In an embodiment the laser light source has a peak power determined atthe exit of the laser light source which is at least about 5 kW, such asat least about 10 kW, such as at least about 30 kW, such as at leastabout 50 kW.

The laser light source is advantageously a mode-locked laser lightsource. In an embodiment laser light source is an actively mode lockedlaser. In an embodiment the laser light source is a passively modelocked laser. The mode locked laser preferably comprises one or moreamplifiers.

In an embodiment the hollow core PCF is configured for guidinglight—preferably single mode light-comprising at least one wavelength inthe range from about 200 nm to about 4.5 μm, preferably at least onewavelength in the range from 1000 nm to about 1100 nm.

In an embodiment the hollow core PCF is configured for guiding acontinuum of light wavelengths, preferably spanning over at least about0.1 μm, such as at least about 0.3 μm, such as at least about 0.5 μm.

In an embodiment the hollow core PCF has a first fiber end which isadapted for being connected to the user apparatus and a second fiber endoptically connected to an output fiber of the laser light source via afiber coupling structure. The fiber coupling structure preferablyprovides a protection of the first fiber end and preferably of a facetof the first fiber end to ensure that the facet and or the hollow coreis not contaminated with dust, moisture or similar. Further to ensure asafe low loss connection the coupling structure may comprise a lens,such as a focusing lens, a graded-index element or other elements suchas it is known in the art for coupling structures for hollow core fibersin general.

In an embodiment the fiber coupling structure comprises a focusing lens,for example such as it is described in U.S. Pat. No. 8,854,728.

In an embodiment the fiber coupling structure comprises a graded-indexelement (GRIN) such as it is described in US 2003/0068150.

In an embodiment the fiber coupling structure comprises a protectionelement such as it is described in U.S. Pat. No. 7,373,062.

In a preferred embodiment the first fiber end is mounted in a ferrulestructure, preferably with a ferrule structure as described in PA 201570876 DK with the title “PHOTONIC CRYSTAL FIBER ASSEMBLY” and whichapplication in hereby incorporated by reference into this disclosurewith the proviso that in case of any inconsistence between the subjectmatter explicitly disclosed herein and the incorporated subject matter,the content of the subject matter explicitly disclosed herein prevail.

The hollow core PCF as described herein may advantageously be producedby drawing from a preform wherein the drawing is performed whilecontrolling the pressure within the hollow tubes for example asdescribed in U.S. Pat. Nos. 6,954,574, 8,215,129, 7,793,521 and/or in PA2016 70262 DK with the title “A RING ELEMENT FOR A PREFORM, A PREFORMAND AN OPTICAL FIBER DRAWN FROM THE PREFORM”

The preform is advantageously produced by providing a hollow tube forthe outer cladding region and 7 hollow tubes for the inner cladding,wherein the hollow tube for the outer cladding region has an innerdiameter which is larger than two times, such as at least 3 times, suchas at least 4 times larger than the outer diameter of the 7 hollowtubes—or if different—of the largest outer diameter of the 7 hollowtubes for the inner cladding. Preferably the outer cladding region hasan inner diameter which is sufficiently large to arrange the 7 hollowtubes inside and in contact with the hollow tube for the outer claddingregion such that the 7 hollow tubes are not touching each other. Toarrange the 7 hollow tubes inside the hollow tube for the outer claddingregion a short section of support elements (e.g. tubes or rods of glass)may be arranged at each end of the hollow tubes. After having fused the7 hollow tubes to the hollow tube for the outer cladding region, theends of the fused hollow tubes comprising the support elements may becut off.

The preform may then be drawn in a fiber drawing tower preferably bysimultaneously individual or common pressure control of the pressurewithin the respective hollow tubes.

In an embodiment the pressure control of each of one or more of thehollow tubes of the preform, also referred to as “elongate holes” ormerely “holes”, is provided by arranging a pressure tube between thehole and a pressure supply. The pressure supply ensures that thepressure within the hollow tube is controlled to a desired level via thepressure tube during the drawing of the fiber from the preform.

The pressure tube is advantageously a hollow pressure tube. The phrase“in the tube” refers to in the hollow part of the pressure tube.

Thus in an embodiment the method comprises inserting a first end of apressure tube into the hole of the preform at a first end of the preformand subjecting the hole of the preform to a controlled pressure via thepressure tube during drawing. Advantageously at least a pressure tubelength section comprising the first end of the pressure tube is insertedinto the elongate hole of the preform. The pressure tube length sectionshould preferably have a length of at least about 0.5 mm, such as fromabout 1 mm to about 20 cm, such as from about 2 mm to about 5 cm, suchas from 0.5 to 1 cm.

In practice it is desired that the pressure tube length section has alength which is sufficient to provide a seal between the pressure tubelength section and the elongate hole and yet it should not be too longsince the length part of the preform comprising the pressure tube lengthsection may in an embodiment not be drawn to fiber or if—in analternative embodiment—it is drawn to fiber, the resulting fiber willthen have different characteristics than the fiber drawn from preformmaterial without the tube length section(s)

It has been found that a safe gas connection may be obtained between theelongate hole and the pressure tube by simply inserting the first end ofthe pressure tube into the hole and optionally providing that thepressure tube length section is expanded such that its outer surfacefits to the periphery surface of the elongate hole and/or applying asealing material, such as glue, epoxy, grease and/or rubber or any otherpliable sealing material.

The pressure tube advantageously has an outer diameter and peripheryselected such that it fits into the hole of the preform. The crosssectional shape of the hole may be round or oval or any other suitableshape. The surface defining the hole is also referred to as theperiphery surface of the elongate hole. The pressure tube preferably hasan outer cross sectional shape corresponding to the cross sectionalshape of the hole of the preform, however with an average diameterslightly smaller than the average diameter of the hole of the preform,such that the first end of the pressure tube can be inserted into thehole.

In an embodiment the pressure tube or at least the pressure tube lengthsection of the pressure tube has an average outer diameter which is fromabout 80% up to 100% of an average inner diameter of the hole, such asfrom about 90% to about 99% of the average diameter of the hole.

Advantageously the pressure tube has a supply section which is outsidethe hole i.e. the part of the pressure tube which leads from the hole toa gas connection to the pressure supply.

In an embodiment the supply section of the pressure tube has a supplyopening which is in gas connection with a pressure source forcontrolling the pressure within the hole.

In an embodiment the supply section of the pressure tube has a supplyopening which is within a pressure regulating chamber. By regulating thepressure within the pressure regulating chamber e.g. by the pressuresource, the pressure within the pressure tube is also regulated andthereby the pressure within the elongate hole is regulated.

In an embodiment the supply section of the pressure tube has a supplyopening which is directly connected to a pressure source for regulatingthe pressure within the pressure tube and thereby the pressure withinthe elongate hole is regulated.

The pressure tube can in principle be of any material. In an embodimentthe pressure tube is of a thermo-moldable material, e.g. a materialwhich may be molded or drawn at a fiber drawing tower. Advantageouslythe pressure tube is of silica optionally comprising a polymer coating.In an embodiment at least the supply section of the pressure tube has anouter polymer coating and optionally the pressure tube length section isfree of polymer coating. The polymer coating increases the pliability ofthe pressure tube and reduces the risk of rupture of the pressure tube.

By providing the pressure tube of silica with a pressure tube lengthsection with an uncoated pressure tube length section and a pressuretube supply section with a polymer coating, a desirably large hollowpart cross-sectional diameter of the pressure tube may be obtained whileat the same time the pressure tube supply section may be desirablypliable and rupture resistant.

As mentioned above several holes of the preform may be pressurecontrolled with pressure tubes as described above. The pressure tubesmay be connected to the same pressure supply or to different pressuresupplies.

All features of the invention and embodiments of the invention asdescribed above including ranges and preferred ranges can be combined invarious ways within the scope of the invention, unless there arespecific reasons not to combine such features.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional objects, features and advantages of thepresent invention will be further elucidated by the followingillustrative and non-limiting detailed description of embodiments of thepresent invention, with reference to the appended drawings.

FIG. 1 shows a cross-section of an embodiment of a hollow core PCF ofthe invention where one of the hollow tubes is enlarged to show theinner diameter d1, the outer diameter d2 and the wall thickness t.

FIG. 2 shows a cross-section of an embodiment of a hollow core PCF ofthe invention where some of the hollow tubes have a larger wallthickness t than other of the hollow tubes.

FIG. 3 shows a cross-section of an embodiment of a hollow core PCF ofthe invention where the hollow tubes are oval.

FIG. 4 shows a cross-section of an embodiment of a hollow core PCF ofthe invention where the hollow tubes comprise nested sub tubes arrangedin the hollow structure of hollow tubes and fused to the hollow tubes.

FIG. 5 shows a cross-section of an embodiment of a hollow core PCF ofthe invention where the hollow tubes comprise nodules arranged at a corecenter facing region.

FIG. 6 shows a cross-section of an embodiment of a hollow core PCF ofthe invention where single mode light has been fed to the hollow corePCF.

FIG. 7 is a graph showing several low loss transmission bands of ahollow core PCF of an embodiment of the invention.

FIG. 8 is a graph showing a broad low loss transmission band of a hollowcore PCF of an embodiment of the invention.

FIG. 9 is a graph showing the suppression of HOMs of a prior art hollowcore PCF with six hollow tubes.

FIG. 10 is a graph showing the suppression of HOMs of hollow core PCF ofan embodiment of the invention with seven hollow tubes.

FIG. 11 is a schematic drawing of a laser system of an embodiment of theinvention as well as a user apparatus.

FIG. 12 illustrates a first end of a preform for an embodiment of ahollow core PCF of the invention with 7 hollow tubes forming 7 elongateholes and where the first ends of the respective pressure tubes areinserted into respective holes for pressure controlling of the holesduring drawing.

FIG. 13 a illustrates a hollow core PCF of an embodiment of theinvention comprising an outer cladding region comprising a photonic bandgap structure.

FIG. 13 b is a graph showing the transmission loss for two variations ofthe hollow core PCF of the embodiment FIG. 13 a.

FIG. 14 is a graph showing a measurement of the beam quality M² of thefiber fabricated in EXAMPLE 1.

DESCRIPTION OF EMBODIMENTS

The figures are schematic and simplified for clarity. Throughout, thesame reference numerals are used for identical or corresponding parts.

The hollow core PCF of the invention shown in FIG. 1 comprises an outercladding region 1 and seven hollow tubes 2 surrounded by said outercladding region. Each of the hollow tubes 2 is fused at fusing point 3to the outer cladding 1 to form a ring defining an inner cladding regionand a hollow core region 4 surrounded by the inner cladding region andhaving a core diameter D.

The hollow tubes are not touching each other and are generally referredto as non-touching hollow tubes. As shown in the enlargement of a hollowtube, each of the hollow tubes 2 has an average outer diameter d2 and anaverage inner diameter d1 and a wall thickness t in the center facingregion of the respective hollow tubes 2. The outer cladding 1 has aninner diameter ID and an outer diameter OD. In this embodiment thehollow tubes 2 are identical and are substantially circular in crosssection.

In the embodiment shown in FIG. 2 the hollow core PCF comprises an outercladding region 11 and seven non-touching hollow tubes 12 a, 12 bsurrounded by said outer cladding region where the hollow tubes 12 a, 12b are fused at fusing point 13 to the outer cladding 11 to form a ringdefining an inner cladding region and a hollow core region 14 surroundedby the inner cladding region. Three of the non-touching hollow tubes 12a have a larger wall thickness t than the remaining four of thenon-touching hollow tubes 12 b. As shown each of the respective sevennon-touching hollow tubes 12 a, 12 b is uniform in thickness, therebyproviding a simpler production of the hollow core PCF.

In the embodiment shown in FIG. 3 the hollow core PCF comprises an outercladding region 21 and seven identical non-touching hollow tubes 22surrounded by said outer cladding region, and the hollow tubes 22 arefused to the outer cladding 21 to form a ring defining an inner claddingregion and a hollow core region 24 surrounded by the inner claddingregion. The hollow tubes 22 are oval and each of the hollow tubes 22 hasa long inner diameter D_(l) and a short inner diameter D_(s)perpendicular to the long inner diameter D_(l), wherein D_(l) isdetermined in radial direction.

In the embodiment shown in FIG. 4 the hollow core PCF comprises an outercladding region 31 and seven identical non-touching hollow tubes 32surrounded by said outer cladding region, and the hollow tubes 32 arefused at fusing points 33 to the outer cladding 31 to form a ringdefining an inner cladding region and a hollow core region 34 surroundedby the inner cladding region. The hollow tubes 32 comprise nested subtubes 35 arranged in the hollow structure of the hollow tubes and fusedto the hollow tubes at the fusing points 33 at their respective maximalradial distance to the core center axis.

In the embodiment shown in FIG. 5 the hollow core PCF comprises an outercladding region 41 and seven non-touching hollow tubes 42 a, 42 bsurrounded by said outer cladding region where the hollow tubes 42 a, 42b are fused to the outer cladding 41 to form a ring defining an innercladding region and a hollow core region 44 surrounded by the innercladding region. Four of the non-touching hollow tubes 12 a are uniformin thickness of their respective walls whereas the remaining three ofthe non-touching hollow tubes 42 b comprise nodules 45 arranged at acore center facing region of the respective hollow tubes. Since theouter diameter d2 of the respective non-touching hollow tubes 42 a, 42 bis substantially identical, the nodules 45 are arranged at a boundary ofthe hollow core region.

In the embodiment shown in FIG. 6 the hollow core PCF mainly has astructure as the hollow core PCF of FIG. 1 . A not shown single modelaser source is arranged to laser light to the PCF at wavelength in alow loss transmission band of the PCF. After 5 m or even 10 the beamtransmitted in the PCF as indicated with ref. 6 has a Gaussian beamquality and is fully single mode. After 10 m of transmission the loss inthe fundamental mode is very low e.g. with a transmission efficiency ofmore than 85%, such as more than 90%.

The graph of FIG. 7 shows several low loss transmission bands of a fiberwith the structure shown in FIG. 1 and a core size D of about 30 μm andt=750 nm. Bands are numbered from longest wavelength band to shorterwavelength bands with increasing number. As seen the hollow core PCF hasfour bands of low loss transmission, wherein 3 of the low losstransmission bands comprise wavelengths below 1.2 μm.

The graph of FIG. 8 shows a closer view of a transmission loss of thehollow core PCF of FIG. 1 with D=30 μm and t=750 nm of transmission bandII. It can be seen that the low loss transmission band is very broadwith a bandwidth of 200-250 nm around 1.0641 μm.

A simulation was made for a hollow core PCF with six hollow tubes (priorart hollow core PCF) and with a hollow core PCF with seven hollow tubes(an embodiment of the invention). The simulation was done with D=30 μm,t=750 nm, @1032 nm.

FIGS. 9 and 10 show HOM extinction ratio relative to d/D (D=Dcore) ofthe prior art hollow core PCF with six hollow tubes (FIG. 9 ) and thehollow core PCF of an embodiment of the invention with seven hollowtubes (FIG. 10 ).

As it can be seen in FIG. 10 an optimal ratio d/D between 0.6 and 0.75ensures that Lp11-like modes and several other HOMs of higher order areresonantly coupled to cladding modes.

In FIG. 9 it can be seen that use of six hollow tubes allows achieving apartly suppression of the Lp11-like modes. However, modes with a higherazimuthal number remain unperturbed limiting the overall HOM extinction.

Thus it is clearly shown that the hollow core PCF of the invention withseven hollow tubes has much improved properties for suppression of HOMsthan the prior art hollow core PCF with six hollow tubes.

Further comparison between hollow core PCF of an embodiment of theinvention and hollow core PCF having six hollow tubes are disclosed inthe article “Hollow-core fibers for high power pulse delivery” byMichieletto et al. Optics Express 7103-7119, March 2016. The content ofthis article is hereby incorporated by reference into this disclosurewith the proviso that in case of any inconsistence between the subjectmatter explicitly disclosed herein and the incorporated subject matter,the content of the subject matter explicitly disclosed herein prevail.

The laser system shown in FIG. 11 comprises a laser light source 51 anda fiber delivery cable 52 for delivering light from the laser lightsource 51 to a user apparatus 54. The fiber delivery cable 52 comprisesas its waveguide a hollow core PCF as described above with one or morelow loss transmission bands correlated to the user apparatus. Asindicated the fiber delivery cable 52 may be rather long while stilldelivering single mode light with high efficiency and low loss in thefundamental mode to the user apparatus 54. The fiber delivery cable 52has a first end 53 a and a second end 53 b. In the shown embodiment eachof the first end 53 a and the second end 53 b are mounted in a ferrulestructure for connecting respectively to the user apparatus 54 and thelaser light source 5.

In an alternative not shown embodiment the second end of the fiberdelivery cable 52 is spliced to a fiber output of the laser light source51.

The preform shown in FIG. 12 is a preform for an embodiment of thehollow core fiber as described herein.

The preform comprises a preform outer cladding region 165 and 7 hollowpreform tubes 161 a, 161 b arranged in a non-touching ring (i.e. thetubes are not touching each other) surrounded by and fused to thepreform outer cladding region 165.

Pressure tubes 164 are arranged to connect each of three of the preformtubes 161 a (primary hollow tubes) to a not shown pressure supply forcontrolling the pressure in the primary hollow tubes 161 a duringdrawing. A pressure tube length section 164 a inserted into the hole ofeach primary hollow tube 161 a is advantageously uncoated silica,whereas the remaining part of the pressure tube 164, referred to as thepressure tube supply section is polymer coated silica. The pressure inthe secondary hollow tubes 161 b may advantageously be controlled in apressure chamber such as shown in FIGS. 15 and 16 of PA 2016 70262 DKwith the title “A RING ELEMENT FOR A PREFORM, A PREFORM AND AN OPTICALFIBER DRAWN FROM THE PREFORM”.

In the embodiment shown in FIG. 13 a the hollow core PCF comprises anouter cladding region 171 and seven non-touching hollow tubes 172forming an inner cladding region and surrounded by said outer claddingregion 171. The hollow tubes 172 are fused at fusing points 173 to theinner side of the outer cladding 171 to form the inner cladding regionand a hollow core region 174 surrounded by the inner cladding region.The outer cladding region comprises a photonic band gap structure in theform of microstructures (inclusions) 175 a having different index thanthe cladding background material 175 b.

The photonic bandgap (PBG) structure may be provided by any means, e.g.by providing the outer cladding region with an index grating comprisingconcentric rings with different refractive index and/or by including.

The inclusions 175 a in the outer background material 175 b are arrangedin a cross-sectional pattern comprising about 5 rings.

As mentioned it is preferred that the inclusions are voids or of gas andhave a relatively small diameter and are arranged with short foroptimizing (minimizing) confinement loss of the desired wavelength orrange of wavelength.

FIG. 13 b shows the transmission loss for two variations of the hollowcore PCF of the embodiment FIG. 13 a . wherein the two variations of thehollow core PCF with PBG structure in their respective cladding havebeen optimized for reducing the confinement loss around a desired centerwavelength of respectively 1064 nm (left) and 1550 nm (right). As it canbe seen the loss is very low. Further it is found that this approach toreduce the confinement loss can be also be exploited in polarizationmaintaining antiresonant fibers.

Further it was found that the confinement loss appears to be arbitraryreduced by increasing the total thickness of the photonic band gapstructure.

Example 1

A hollow core PCF having a structure as shown in FIG. 1 was fabricatedusing the stack and raw technique. The fabricated fiber shown has a corediameter of approximately 30 μm, d2 ^(˜)17 μm with a d2/D^(˜)0.57. Themode field diameter measured at 1064 nm is 2 2 μm. The tubes presentminor size differences, nonetheless the fiber shows remarkable low lossand bend loss and good mode quality. The mode quality factor of thefabricated fiber was measured the mode quality factor with acamera-based M² measurement system (Spiricon M2-2OOS) with a laser at awavelength of 1064 nm and a 5 m FUT. We performed the measurements: withthe fiber coiled on a standard 8 cm spool and no further coils. Theresults are summarized in Table I and shown in FIG. 14 . The fiberoutput beam presents negligible astigmatism and asymmetry and a M² of1.2.

TABLE 1 Asymmetry Astigmatism M² X M² Y 1.02 0.01 1.22 1.2

EMBODIMENTS

1. A hollow core photonic crystal fiber (PCF) comprising an outercladding region and 7 hollow tubes surrounded by said outer claddingregion, wherein each of said hollow tubes is fused to said outercladding region to form a ring defining an inner cladding region and ahollow core region surrounded by said inner cladding region, whereinsaid hollow tubes are not touching each other.

2. The hollow core PCF of embodiment 1, wherein said hollow tubes eachhave an average outer diameter d2 and an average inner diameter d1,wherein d1/d2 is equal to or larger than about 0.8, such as equal to orlarger than about 0.85, such as equal to or larger than about 0.9.

3. The hollow core PCF of embodiment 1 or embodiment 2, wherein saidhollow tubes have a center to center distance A between adjacent hollowtubes which is between about 1.01*d2 and about 1.5*d2, such as between1.05*d2 and 1.2*d2.

4. The hollow core PCF of any one of the preceding embodiments, whereinsaid hollow tubes have substantially parallel center axes.

5. The hollow core PCF of any one of the preceding embodiments, whereinsaid hollow core region has a core diameter D of from about 10 μm toabout 100 μm, such as from about 10 am to about 60 μm, such as fromabout 20 am to about 50 μm, such as from about 25 am to about 40 μm.

6. The hollow core PCF of embodiment 5, wherein said average outer tubediameter relative to the core diameter d2/D is from about 0.5 to about0.75, such as from about 0.65 to about 0.72.

7. The hollow core PCF of any one of the preceding embodiments, whereineach of said hollow tubes comprises a core center facing region with awall thickness t, wherein said wall thickness is up to about 2.1 μm,such as up to about 1 μm, such as in the range from about 150 to about350 nm or in the range from about 650 to about 850 nm or in the rangefrom about 900 to about 2.1 μm.

8. The hollow core PCF of embodiment 7, wherein said wall thickness t ofeach of said hollow tubes is substantially identical, preferably saidhollow tubes are substantially identical and are arranged with identicaldistance to adjacent hollow tubes.

9. The hollow core PCF of embodiment 7, wherein at least one of thehollow tubes has a different wall thickness t than at least one other ofthe hollow tubes, preferably 3 of the hollow tubes have a wall thicknessand the remaining 4 hollow tubes have another wall thickness.

10. The hollow core PCF of embodiment 9, wherein at least one of thehollow tubes has a wall thickness which is at least about 5% larger thanthe wall thickness of at least one other of the hollow tubes, preferablyat least one of the hollow tubes has a wall thickness which is at leastabout 10% larger than the wall thickness of at least one other of thehollow tubes.

11. The hollow core PCF of any one of the preceding embodiments, whereineach of said hollow tubes is substantially circular.

12. The hollow core PCF of any one of the preceding embodiments, whereineach of said hollow tubes has a long inner diameter D_(l) and a shortinner diameter D_(s) perpendicular to the long inner diameter D_(l),wherein D_(l) is determined in radial direction.

13. The hollow core PCF of any one of the preceding embodiments, whereinat least one of said hollow tubes comprises a nested sub tube arrangedin the hollow structure of said hollow tube and fused to said hollowtube, said sub tube has an average outer diameter d_(sub), which issubstantially smaller than said average inner diameter d of said hollowtube, said average outer diameter d2 _(sub) is preferably up to about0.9*d2 of said hollow tube, such as up to about 0.9*d2, preferably saidinternal sub tube is fused to said hollow tube at its maximal radialdistance to the core center axis.

14. The hollow core PCF of any one of the preceding embodiments, whereinat least one of said hollow tubes comprises one or more nodules arrangedat a core center facing region of one or more of the hollow tubes,preferably said nodules are arranged at a boundary of the hollow coreregion, said nodules are preferably arranged to be anti-resonant at anoperating wavelength, so that light of a fundamental mode issubstantially excluded from the nodules.

15. The hollow core PCF of any one of the preceding embodiments, whereinthe minimum distance between adjacent hollow tubes is at least about 0.1μm, such as at least about 1 μm, such as at least about 2 μm, such as atleast about 5 μm.

16. The hollow core PCF of any one of the preceding embodiments, whereinthe hollow core region and said hollow tubes independently of each othercomprise gas selected from air, argon, nitrogen or mixtures comprisingany of the mentioned gasses, optionally said hollow core region and saidhollow tubes independently of each other are vacummated or filled withpressurized gas.

17. The hollow core PCF of any one of the preceding embodiments, whereinthe outer cladding region has an outer diameter of at least about 125μm, 150 μm, such as at least about 200 μm.

18. The hollow core PCF of any one of the preceding embodiments, whereinthe outer cladding region and/or the hollow tubes comprise a solid glassmaterial, preferably said solid material is of silica, optionally dopedwith refractive index modifying dopant.

19. The hollow core PCF of any one of the preceding embodiments, whereinthe outer cladding region comprises a photonic bandgap structuresurrounding said inner cladding region.

20. The hollow core PCF of embodiment 19, wherein the outer claddingregion comprises a outer background material having a refractive indexN_(oc) and a plurality of inclusions having a refractive index differentfrom the refractive index of the background material, said plurality ofinclusions in the outer background material is preferably arranged in across-sectional pattern comprising at least two rings of inclusionssurrounding the inner cladding region, such as at least 3 rings, such asat least 4 rings of inclusions.

21. The hollow core PCF of embodiment 20, wherein the plurality ofinclusions in the outer background material is arranged in asubstantially hexagonal pattern.

22. The hollow core PCF of embodiment 20 or embodiment 21, wherein theplurality of inclusions are of solid material, such as down doped solidmaterial e.g. doped with fluorine and/or boron.

23. The hollow core PCF of embodiment 20 or embodiment 21, wherein theplurality of inclusions are voids or of gas, such as air.

24. The hollow core PCF of any one of embodiments 20-23, wherein saidplurality of inclusions have an average diameter (d_(inc)) of up toabout 2.5 μm, such as up to about 2 μm, such as between about 1.1 μm and1.8 μm, such as between about 1.15 am and about 1.7 μm, such as betweenabout 1.2 μm and about 1.5 μm, such as about 1.3 μm.

25. The hollow core PCF of any one of embodiments 20-24, wherein saidplurality of inclusions are arranged at a pitch (Λ_(inc)) of up to about6 μm, such as up to about 5 μm, such as up to about 4 μm, such asbetween about 2 μm and 4 am.

26. A laser system for delivering laser light to a user apparatus, saidlaser system comprising a laser light source and a fiber delivery cablefor delivering light from the laser light source to the user apparatus,wherein said fiber delivery cable comprises a hollow core PCF accordingto any one of the preceding embodiments.

27. The laser system of embodiment 26, wherein said laser light sourceis configured for generating laser light pulses and is opticallyconnected to said fiber delivery cable, preferably said laser lightsource is a femtosecond laser source.

28. The laser system of embodiment 27, wherein said laser light sourcehas a pump duration of from about 30 fs to about 30 ps, such as fromabout 100 fs to about 10 ps.

29. The laser system of embodiment 27 or embodiment 28, wherein saidlaser light source has a peak power determined at the exit of the laserlight source which is at least about 5 kW, such as at least about 10 kW,such as at least about 30 kW, such as at least about 50 kW.

30. The laser system of any one of embodiments 27-29, wherein said laserlight source is a mode locked laser, such as an actively mode lockedlaser or a passively mode locked laser, said mode locked laserpreferably comprises one or more amplifiers.

31. The laser system of any one of embodiments 26-30, wherein saidhollow core PCF is configured for single mode guiding of at least onewavelength in the range from about 200 nm to about 4.5 μm, preferably atleast one wavelength in the range from 1000 nm to about 1100 nm.

32. The laser system of embodiment 31, wherein said hollow core PCF isconfigured for guiding a continuum of light wavelengths, preferablyspanning over at least about 0.1 μm, such as at least about 0.3 μm, suchas at least about 0.5 μm.

33. The laser system of any one of embodiments 26-32, wherein saidhollow core PCF has a first fiber end which is adapted for beingconnected to said user apparatus and a second fiber end opticallyconnected to an output fiber of said laser light source via a fibercoupling structure.

34. The laser system of embodiment 26, wherein said fiber couplingstructure comprises at least one of

a focusing lens,

a graded-index element (GRIN),

a protection element, or

a ferrule structure.

35. The laser system of embodiment 33 or embodiment 34, wherein saidfirst fiber end fiber is mounted in a ferrule structure.

The invention claimed is:
 1. A hollow core photonic crystal fiber (PCF)comprising an outer cladding region and a plurality of hollow tubessurrounded by said outer cladding region, each of said hollow tubeshaving an average inner tube diameter d₁ and an average outer tubediameter d₂, where each of said hollow tubes is fused to said outercladding region to form a ring defining an inner cladding region and ahollow core region surrounded by said inner cladding region, wherein atleast one of said hollow tubes comprises a nested sub-tube having anaverage outer sub-tube diameter d_(sub) smaller than the average innerdiameter d₁ of said hollow tube and arranged in the hollow structure ofthe hollow tube and fused to the hollow tube, wherein the at least oneof said hollow tubes forms a first tubular element, and wherein thenested sub-tube forms a second tubular element with a third tubularelement nested within the second tubular element, wherein each of saidhollow tubes comprises a core center facing region with a wallthickness, and wherein the hollow core region and/or one or more of theplurality of hollow tubes are evacuated from gas.
 2. The hollow core PCFof claim 1, wherein at least one of the plurality of hollow tubes has adifferent wall thickness than at least one other of the plurality ofhollow tubes.
 3. The hollow core PCF of claim 1, wherein said hollowtubes are not touching each other.
 4. The hollow core PCF of claim 1,wherein said nested sub-tube is fused to said hollow tube at its maximalradial distance to the core center axis.
 5. The hollow core PCF of claim1, wherein said hollow tubes have a center to center distance Λ betweenadjacent hollow tubes which center to center distance is between about1.01*d2 and about 1.5*d2.
 6. The hollow core PCF of claim 1, wherein theminimum distance between adjacent hollow tubes is at least about 0.1 μm.7. The hollow core PCF of claim 1, wherein said hollow core region has acore diameter D defined as the diameter of the largest circle inscribedby the hollow tubes.
 8. The hollow core PCF of claim 7, wherein thehollow core region diameter D is from about 10 μm to about 100 μm. 9.The hollow core PCF of claim 7, wherein said average outer tube diameterrelative to the core diameter d2/D is from about 0.5 to about 0.75. 10.The hollow core PCF of claim 7 wherein said average outer tube diameterrelative to the core diameter d2/D is from about 0.65 to about 0.72. 11.The hollow core PCF of claim 1, wherein said plurality of hollow tubescomprises 7 hollow tubes.
 12. The hollow core PCF of claim 1, whereinsaid plurality of hollow tubes consist of 7 hollow tubes.
 13. The hollowcore PCF of claim 1, wherein the sub-tube average outer diameter drub,is substantially smaller than said average inner diameter d₁ of saidhollow tube.
 14. The hollow core PCF of claim 1, wherein said wallthickness is up to about 2.1 μm.
 15. A laser system for delivering laserlight, said laser system comprising a laser light source and a fiberdelivery cable for delivering light from the laser light source, whereinsaid fiber delivery cable comprises a hollow core PCF according toclaim
 1. 16. The laser system of claim 15, wherein said laser lightsource has a pulse duration of from about 30 fs to about 30 ps and apeak power determined at the exit of the laser light source which is atleast about 5 kW.
 17. The hollow core PCF of claim 1, wherein the hollowcore region and/or one or more of the plurality of hollow tubes areevacuated from gas to a pressure of about 1 mbar or less, at standardtemperature.
 18. The hollow core PCF of claim 1, wherein the hollow coreregion and/or one or more of the plurality of hollow tubes are evacuatedfrom gas to a pressure of about 0.1 mbar or less, at standardtemperature.
 19. The hollow core PCF of claim 1, wherein the hollow coreregion and/or one or more of the plurality of hollow tubes are evacuatedfrom gas to a pressure of about 0.01 mbar or less, at standardtemperature.